Many techniques for compressing data bandwidth in communication systems are known to those skilled in the art. A comprehensive survey of such techniques is given in "Principles of Communication Systems", by Herbert Taub and Donald L. Schilling, published by McGraw-Hill Publishing Co. Two of the best known methods are Quadrature Amplitude Modulation (QAM) and Multiple Phase Shift Keying (MPSK). In the QAM method, a binary input signal is used to modulate the amplitude envelope of a carrier signal while a second digital signal modulates the R.F. signal in phase and in quadrature to the FM signal. Zero crossings between the peaks and dips of the amplitude envelope are detected on the receiving end as bit changes along with measurements of amplitude in order to restore the binary data. However, this method is subject to considerable error in restoring the data if undesired amplitude and phase shifts are detected as bit changes. In the MPSK method, e.g., bit changes in the input data signal are encoded in reformatted pulses according to fixed encoding rules and the R.F. signal is phase modulated by fixed angular phase changes.
Certain pulse width modulated schemes have been used at baseband to modify the spectrum for easier data processing. For example, in the encoding technique which is referred to as MFM or Miller Encoding, output pulses are encoded with widths of 2, 3, and 4 times the clock period (1, 1.5, and 2 times the bit width) with their phases delayed to the end or center of a data bit for "0" or "1", respectively. By suitable filtering, only those frequencies resulting from the defined set of periods need to be detected to restore the original data signal. Unfortunately MFM does not conserve bandwidth when used to modulate an R.F. carrier.
A QAM modulation system affords a transmission improvement of 2 or more times over baseband NRZ or a single sideband carrier system, while it gives an improvement of 4 or more times over the typical AM modulated carrier system. Both of these methods are theoretically capable of bandwidth compression up to a limit of about 10 to 1. However, such compression would be achieved with a considerable loss in signal-to-noise ratio. In order to reduce errors due to signal-to-noise ratio, a 10-to-1 compression ratio typically requires a 100-to-1 increase in power. This is due to the well-known relationship applicable to both the QAM and MPSK methods, i.e., 2.sup.N =M, where N represents compression efficiency (referred to as the Nyquist Factor) expressed as the number of bits per hertz of bandwidth, and M represents the number of modulation levels. For example, a Nyquist Factor of N=10 would require 1024 modulation levels in the above-described conventional methods. The high number of modulation levels would greatly increase the power requirement or seriously degrade the signal-to-noise ratio, and therefore such an increase in Nyquist Factor is barely attainable by the conventional methods.
In the related U.S. Pat. No. 4,742,532 of Harold R. Walker, a new method of modulation is disclosed which is referred to as Variable Phase Shift Keying (VPSK). In the VPSK modulation scheme described in the Walker patent, changes of state between "1" and "0" of a binary NRZ (non-return to zero) input data signal are encoded to pulse signals having varying time periods which are 4/4, 5/4, and 6/4 times the bit period according to a particular set of coding rules. It would appear that this is a variation of MFM as used in double density disk recording but the spectral content of a 2, 3, 4 MFM pattern and the spectral content of a VPSK signal are entirely different. For reasons not easily explained MFM with M=2 does not compress the bandwidth, while increasing the M number to 3 or greater results in compression.
As illustrated in FIG. 1 herein, an NRZ input signal consists of a string of "1" and "0" bits of a given period. An encoder outputs a rectangular wave signal which switches polarity with varying widths depending upon the data bits encountered in the input stream. The encoded signal is given a width of 4/4 bit period when no change of the polarity state from one data bit to the next is detected (bit which is "1" or "0" is repeated), or a width of 5/4 bit periods when a data bit polarity change is detected. The last data bit polarity change is encoded in one of two widths, depending upon the state of the following, last data bit in the encoding sequence. The last data bit in the encoding sequence is then omitted to make up for the longer widths of the encoded signal. A width of 6/4 bit periods is used when the last data bit is a "1" after four data bit polarity changes. When the last bit is a "0" after four data bit polarity changes, no change is made to the encoding period. The "1" or "0" bit will automatically be restored by the decoder program. This method can obtain a Nyquist Factor of 7.2 bits/Hz-bandwidth using 4 modulation levels. Thus, it surpasses the conventional compression efficiency relationship, wherein 4 modulation levels would provide a Nyquist Factor of 2 (2.sup.2 =4).
However, it is desirable to increase the compression efficiency in data communication systems even further, for example, to a Nyquist Factor of 10 bits/Hz-bandwidth or higher. In the conventional QAM and MPSK systems, increasing the number of modulation levels above 4 requires approximately 6 dB or 4 times the power level for each increase of 2 in the Nyquist Factor, otherwise a serious loss in signal-to-noise ratio is encountered. It is therefore a principal object of the invention to provide an improved binary data communication system which produces a large improvement in the Nyquist Factor over the prior methods without a corresponding increase in the power requirement or loss of signal-to-noise ratio.